Power of d Choices for Large-Scale Bin Packing: A Loss Model

We consider a system of $N$ parallel servers, where each server consists of B units of a resource. Jobs arrive at this system according to a Poisson process, and each job stays in the system for an exponentially distributed amount of time. Each job may request different units of the resource from the system. The goal is to understand how to route arriving jobs to the servers to minimize the probability that an arriving job does not find the required amount of resource at the server, i.e., the goal is to minimize blocking probability. The motivation for this problem arises from the design of cloud computing systems in which the jobs are virtual machines (VMs) that request resources such as memory from a large pool of servers. In this paper, we consider power-of-d-choices routing, where a job is routed to the server with the largest amount of available resource among d ≥ 2 randomly chosen servers. We consider a fluid model that corresponds to the limit as N goes to infinity and provide an explicit upper bound for the equilibrium blocking probability. We show that the upper bound exhibits different behavior as B goes to infinity depending on the relationship between the total traffic intensity λ and B. In particular, if (B -- λ)/√λ → α, the upper bound is doubly exponential in √λ and if (B -- λ)/log_d λ → β, β > 1, the upper bound is exponential in λ. Simulation results show that the blocking probability, even for small B, exhibits qualitatively different behavior in the two traffic regimes. This is in contrast with the result for random routing, where the blocking probability scales as O(1/√λ) even if (B -- λ)/√λ → α.